Light, and elementary fire, compose, it is said, only one and the same thing. This may be, but as we have not yet a clear idea of elementary fire we shall desist from pronouncing on this first point. Light and fire, such as we are acquainted with, are two distinct substances, differently composed. Fire is, in fact, very often luminous, but it sometimes also exists without any appearance of light. Fire, whether luminous or obscure, never exists without a great heat, whereas light often burns with a noise without the least sensible heat. Light appears to be the work of nature while fire is only the produce of the industry of man. Light subsists of itself, and is found diffused in the immense space of the whole universe. Fire cannot subsist without food, and is only found in some parts of this space where man preserves it, and in some parts of the profundity of the earth, where it is also supported by suitable food. Light when condensed and united by the art of man, may produce fire, but it is only as much as it lets fall on combustible matters. Light is therefore no more, and in this single instance, only the principle of fire and not the fire itself: even this principle is not immediate, for it supposes the intermediate one of heat, and which appears to appertain more than light to the essence of fire. Now heat exists as often without light as light exists without heat: these two principles might, therefore, appear not to bind them necessarily together; their effects are not contemporary, since in certain circumstances we feel heat long before light appears, and in others we see light long before we feel any heat. Hence is not heat a mode of being, a modification of matter, which, in fact, differs less than all the rest from that of light, but which can be considered apart, and still more easily conceived? It is, nevertheless, certain, that much fewer discoveries have been made on the nature of heat than on that of light; whether man better catches what he sees than what he feels; whether light, presenting itself generally as a distinct and different substance from all the rest, has appeared worthy of a particular consideration; whereas heat, the effect of which is the most obscure, and presents itself as a less detached and less simple object, has not been regarded as a distinct substance but as an attribute of light and fire.